1. Field of the Invention
This invention relates to an improved method for prelubrication treatment of substrates' surfaces via the creation of nanophase layers on the surfaces, and more particularly, this invention relates to an improved method for preparing a functionally graded nanocrystalline layer on a metal surface to improve friction and wear performance, especially resistance to scuffing.
2. Background of the Invention
A problem extant in tribological science (the science of wear, friction, and lubrication) is a surface distress phenomenon commonly known as “scuffing.” Scuffing is a severe adhesive failure situation associated with high speed, high load lubricated contacts. Lack of adequate lubrication may cause localized damage of the metal surfaces, often regarded as being micro-welding. Scuffing is particularly prevalent with cams, tappets, cylinder bores, and gears.
Scuffing often appears as a dull matte finish at the extreme end regions of the contact path. In addition to a line or path, scuffing can also affect an area. Generally, scuffing occurs when the sliding speed of a weight-bearing or loaded contact on a substrate surface exceeds a critical value known as the scuffing resistance.
Scuffing is characterized by direct intermittent surface-to-surface contact through the lubricating oil film. This contact occurs either due to poor entrainment, localized surface roughness, or debris entrapment. The contact causes an increase in the friction and, due to high loads and speeds, an increase in the frictional heating. The frictional heating gives rise to a decrease in the oil viscosity, with a corresponding decrease in the oil film thickness and, inevitably, a higher frequency of surface contact events. The increased contact causes even greater friction between the surfaces, more heating, even lower lubricant viscosity, and decreased separation between the surfaces. The number of surface contact events keeps increasing until the two surfaces suffer sudden, massive adhesive contact and seizure. This produces characteristic heat transformation layers at the surface.
One current method in the art to combat scuffing is nitriding or nitrocarburizing treatments. Another method is the use of low-friction coatings such as diamond-like carbon (DLC).
The scuffing problem and other tribological failures, such as abrasive wear, can be alleviated or even eliminated by reducing the grain size at and near the surface of substrates. Typical initial grain sizes in tribological components range from about 2 to 5 micrometers (μm) or microns (μ). Reducing the grain size of crystalline materials to nanometer (nm) size brings about significant changes in the properties of materials, e.g., increased hardness and yield strength to yield tribological or wear-resistant components. Nanophase or nanocrystalline materials are currently prepared by a plurality of techniques such as rapid solidification, rapid vapor condensation, ball milling, and the sol-gel process. All of the aforementioned processes are expensive for producing large quantities of materials. Further, in many situations, the nanocrystalline formation or property enhancement is needed only on the surface of a substrate.
U.S. Pat. Nos. 6,652,913; 6,620,498; and 6,472,060 awarded to Ruppi, et al. on Nov. 25, 2003, Sep. 16, 2003, and Oct. 29, 2002, respectively, disclose a method for depositing nanocrystalline coatings using chemical vapor deposition (CVD).
U.S. Pat. No. 5,309,874 awarded to Willermet, et al. on May 10, 1994 discloses a method for depositing nanocrystalline coatings using plasma-enhanced chemical vapor deposition (CVD) and sputtering.
None of the aforementioned patents provide for a simple method by which a nanocrystalline layer of more than a few microns thick can be created upon a substrate's surface. Further, none of these patents provide for the creation of the layer containing the same material as that found within the substrate. In addition, none of these patents provide for a mechanical method requiring a minimum of equipment.
A need exists in the art for a method by which nanocrystalline layers can be created by a direct mechanical means. A need also exists to eliminate the distinctive interface between coatings and the underlying substrate material. The method should be inexpensive and readily portable.